The present invention generally relates to the production of corrugated cardboard, and more particularly, to a novel and improved method for accurately applying an adhesive to the flutes of corrugated board centered on the flute crests, so that the flutes can be bonded to a face.
Typically, corrugated cardboard is formed by producing a corrugated sheet which is initially bonded along one side to a single face. Adhesive is then applied to the crests of the flutes remote from the single face by an applicator roll of a glue machine. Thereafter, a second face is applied to the adhesive on the flutes to produce a composite structure in which corrugations extend between and are bonded to spaced-apart faces.
In some instances, multiple-layer cardboard is produced in which more than one corrugated sheet is adhesively attached to additional faces so that, for example, a central flat face is bonded to a corrugated sheet on each side thereof, and outer flat faces are bonded to the sides of the two corrugated sheets remote from the central face.
The corrugated sheet is typically passed between a rider roll and an applicator roll to apply the adhesive to the flutes. The rider roll typically applies sufficient downward pressure to force the flute tips into contact with the applicator roll. This downward pressure causes compression or deformation of the flutes. The flutes enter the adhesive layer prior to being crushed against the applicator and often become overly wetted or saturated with adhesive due to the long dwell time. As a result, the flutes do not return to their original shape after being crushed. This permanent deformation of the flutes reduces the strength of the final cardboard.
It has been known in the art that glue machines can be run with the applicator roll operating at a lower speed than the web speed (speed at which the corrugated sheet passes between the applicator roll and the rider roll) in order to adjust glue weight. Unfortunately, as the applicator roll speed falls below about 98% that of the web speed, the difference in the relative speeds begins to pull or drag adhesive from the flute crests onto the trailing sloped face of the flutes. The result is that adhesive ends up being applied nonuniformly to the flutes, and that the face surfaces of a finished corrugated product are not smooth due to washboarding (i.e. face surface being pulled and adhered into the valleys of the corrugated sheet). The finished corrugated cardboard product is thus weaker due to weak bond strength between the corrugated sheet and the adhered-to face sheet. The finished cardboard also experiences directional differences in strength. Therefore, it has been impractical to adjust glue weight by lowering the applicator roll speed much below the web speed, and applicator roll speeds of at least 98% web speed have become the industry standard.
The adhesive applied to the flutes is also asymmetrical because the flutes plow through the adhesive layer on the applicator and are wetted on one sloped face more than the other. This asymmetrical application of the adhesive results in a lower bond strength for a given weight of adhesive and a rough surface finish on the face sheet due to warpage after the adhesive cures. Additionally, a relatively large amount of over spray is created which further increases the amount of glue used by the process.
Accordingly, there is a need in the art for an improved method for producing corrugated cardboard which obtains maximum strength in the finished product and an improved surface finish on the face. Furthermore, it is desirable to apply substantially less adhesive per unit area of the finished product and to produce the improved cardboard at an increased rate of production. It is particularly desirable to provide a method of applying adhesive accurately and sparingly to the centers or crests of the corrugated flutes without significant adhesive being applied to either the leading or trailing sloped faces of the flutes. Most preferably, such a method will allow glue weight adjustment by operating the applicator roll substantially below the web speed, preferably less than 95% the web speed, while still providing the adhesive only to the flute crests.
The present invention provides a method and apparatus for uniformly and accurately applying adhesive to the crests of the flutes of corrugated sheets with little or no (or substantially no) adhesive being applied to either the leading or trailing sloped faces of the flutes. In accordance with the present invention, higher line speeds can be achieved, tighter performance specifications exceeding the capability of the industries standard machines are possible, and a significant reduction in the amount of glue used is achieved. In addition, accurately centering the adhesive onto the crests of the flutes provides stronger bond strength between the corrugated sheet and the adhered-to face sheet. Directional differences in strength are minimized or substantially eliminated, and surface smoothness of the face sheets is improved (washboarding reduced). Because the adhesive is very accurately deposited only to the flute crests, it is possible to reduce the adhesive weight deposition rate about 10-70% of that required in conventional machines while delivering the same or comparable bond and crush strength. Additionally, because there is no practical lower limit to the controlled glue weight, cold set adhesives can be used to further improve board properties and reduce energy costs and warpage losses. Furthermore, in accordance with the present invention, smoother and more printable boards with greatly reduced warpage and improved surface finish are produced.
A preferred method according to the present invention includes the steps of rotating an applicator roll having an adhesive layer on the surface thereof on a rotational axis, and rotating a rider roll on a rotational axis substantially parallel, and located at a height substantially equal, to that of the rotational axis of the applicator roll. The corrugated sheet travels along a vertical path between the applicator roll and the rider roll, engaging the crests with the adhesive layer on the applicator roll to apply adhesive to the crests. The flutes are compressed against the applicator roll by the rider roll to achieve a height compression of 3-30% of the initial flute height as the adhesive is applied (i.e. the flutes are compressed down to 70-97% of their initial flute height) Most preferably, the flutes are compressed 5-15%, i.e. down to 85-95% of their initial flute height.
According to a further aspect of the invention, the method includes the steps of providing the layer of adhesive on the applicator roll, and moving the corrugated sheet through a space between the applicator roll and the rider roll for engaging the flutes with the adhesive layer on the applicator roll to apply adhesive to the flutes. The corrugated sheet travels past the applicator roll at a first speed and the applicator roll is rotated at second speed, such that a surface linear velocity of the applicator roll is less than 95% of the first speed, preferably less than 50% of the first speed, preferably less than 45% of the first speed, most preferably less than 40% of the first speed.